Effects of adaptation to mountain altitude of 3200 m on the contractile function and adrenoreactivity of the heart in rats have been investigated. It was shown that adaptation to altitude increased the contractile force and contraction and relaxation velocities of the left ventricle as compared to the controls. Simultaneously, a significant increase in cardiac response to noradrenaline developed in the course of adaptation. The increased response was accompanied by its more rapid disappearance. These changes may be explained by the increase in the activity of myocardial adenylate cyclase and phosphodiesterase. It was also shown that the decrease in cardiac function produced by cardiac denervation was less pronounced in adapted rats. This fact may be explained by increased effectiveness of cardiac autoregulatory mechanism. The comparison of these results with the data of other investigators suggests that in well adapted animals the strength of interaction between the levels of the regulatory hierarchy of the whole organism changes, i.e., the capacity of autoregulatory cell mechanisms and their reactivity to neurohumoral stimuli increase. As a result, control of the organism's reactions by higher levels of the regulatory system is more economical in adapted animals.